The present invention relates to a pit prop assembly. More particularly this invention concerns such an assembly comprising a pair of pit props adapted to be connected to a face conveyor extending along the mine face.
A pit prop assembly frequently comprises a pair of pit props having roof-engaging arms each pivoted on a respective floor-engaging base. A hydraulic ram extends between each base and the respective arm so as to press the arm against the roof and the base against the floor, thereby locking the pit prop in place and holding the roof up to prevent cave-ins. The face conveyor on which is provided the guide along which the face-working device, e.g. a coal plow, moves is connected via a lifting or positioning mechanism to the front end of floor-engaging slide between each pair of bases. This slide is elongated and extends perpendicularly to the face. The rear end of the slide is connected via a hydraulic cylinder to the front ends of the bases. The purpose of this last-mentioned cylinder is to displace the pit props toward or away from the face when their rams are relaxed, or to displace the conveyor toward or away from the face when the pit props are locked between the floor and the roof.
Normally the piston rod of the longitudinal-displacement ram extending between the front ends of the bases and the rear end of the slide is connected to the bases and the cylinder is connected to the slide. This allows the large surface area of the rear face of the piston to be effective in displacement in the pit props toward the face. Similarly the annular space around the piston rod on the other face of the piston is used to stabilize the slide when a face-working device is operating on the face from the front of the face conveyor. Thus, this smaller surface area of the piston alone serves to prevent the slide from pivoting up and down or rocking on its front end as considerable vertical stresses are applied to this front end at the face conveyor. It is therefore necessary to maintain considerable hydraulic pressure at this face of the piston in order to hold the face conveyor and the slide still.
Another disadvantage of this known system is that the considerable lever arm constituted by the slide often tends to press the front or rear end of the slide into the mine floor, especially in cases where a soft spot is under the pit props. Such a settling of the slide can lead to considerable damage of the face conveyor if it is not properly compensated or in the positioning mechanism for this conveyor.
It is possible to overcome these various difficulties by overdimensioning the longitudinal displacement ram and slide. This has the effect of making the device very bulky and quite expensive. In addition the often enormous forces exerted on the slide at the conveyor are still occasionally able to damage the pit props by over-stressing the slide and longitudinal-displacement ram.